Image signal processor for reading dark and light characters from light and dark backgrounds respectively

ABSTRACT

An image signal processor is capable of precisely reading a dark-colored character or the like drawn on a light-colored background, such as a white board, and to perform binary encoding, but also capable of precisely reading a light-colored character or the like drawn on a dark-colored background, such as a blackboard, and to perform binary encoding. The image signal processor includes an automatic gain control circuit for receiving an image signal generated by an image pickup element and for changing to either a first cutoff frequency for performing automatic gain control on only a low-frequency component of the signal or a second cutoff frequency for performing automatic gain control on both the low-frequency component and a high-frequency component of the image signal. A binary encoding circuit receives an output signal generated as a result of the automatic gain control operation in the automatic gain control circuit and compares the output signal with a threshold potential level; to generate a binary signal when the threshold potential level is lower than the potential level of a frequency component of the output signal. A separate threshold potential level and frequency component is used when the different background modes are selected. A gate circuit which is operated in conjuction with the change of the cutoff frequency is used to select the binary signal from the binary encoding circuit corresponding to the selected background mode.

BACKGROUND OF THE INVENTION

The present invention relates to an image signal

processor for reading and printing a character or a pattern drawn on awhite board, a blackboard, or the like.

There exists an image pickup and printing apparatus which is used in amanner such that a character, a pattern, or the like which is drawn on ascreen, such as a white board, is projected through a lens while beingmonitored through a viewfinder which determines the angle of view forthe projected object. An image of the projected object is created by animage pickup element, such as a charge-coupled device, which generatesan image signal to operate a printer for printing the image on paper.That apparatus is mainly used to read a character, a pattern, or thelike drawn in a dark color, such as black or red, on a light-colorbackground, such as a white board. That apparatus cannot be directlyused to read a character, a pattern, or the like drawn in a light color,such as white or yellow, on a dark-colored background, such as ablackboard.

To accomplish the reading operations in the existing apparatus, thebackground and the characters patterns are converted intomutually-reversed binary signals with respect to an image signalsupplied from the image pickup element. The image signal is usuallysubjected to automatic gain control to stabilize an output signal. Forthat reason, when that apparatus is used to read the characters or thelike drawn in a light color on a dark-colored background, the potentiallevel of an output signal component for the dark-colored background istoo high to perform binary encoding, as shown in FIG. 4.

When the existing apparatus is used to read the characters or the likedrawn in a dark color on a light-colored background, the potential levelof an image signal component for the characters or the like is lowerthan the potential level of an image signal component for thebackground. The image signal component for the background is, therefore,subjected to the automatic gain control so as to set the potential levelof an output signal component near the saturation level of an amplifier,thereby making the amplitude of an output signal component for thecharacter or the like large.

On the other hand, when the existing apparatus is used to read thecharacters or the like drawn in a light color on a dark-coloredbackground, an image signal component for the background is subjected tothe automatic gain control to set the potential level of an outputsignal component for the background near the saturation level of theamplifier. In that case, however, since the potential level of an imagesignal component for the character or the like is higher than thepotential level of the image signal component for the background, theamplitude of an output signal component for the character or the likecannot be made large, as shown in FIG. 4.

For that reason, it is difficult to compare the potential level of theoutput signal component for the character or the like with the potentiallevel of a comparison signal to perform binary encoding. Therefore,although the existing apparatus can be used to read a dark-coloredcharacter or the like drawn on a light-colored background, thatapparatus cannot be directly used to read a light-color character or thelike drawn on the dark-colored background, thereby establishing aproblem with that apparatus.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animage signal processor which is capable of precisely reading adark-colored character or the like drawn on a light-colored background,such as a white board, to perform binary encoding, as well as capable ofprecisely reading a light-colored character or the like drawn on adark-colored background, such as a blackboard, to perform binaryencoding.

The image signal processor comprises an automatic gain control circuitfor receiving an image signal generated by an image pickup element andfor changing to either a first cutoff frequency for performing automaticgain control on only a low-frequency component of the signal or a secondcutoff frequency for performing automatic gain control on both thelow-frequency component and a high-frequency component of the imagesignal; a first binary encoding circuit for receiving an output signalgenerated as a result of the automatic gain control in the automaticgain control circuit and for comparing the output signal with a firstthreshold potential level; the first binary encoding circuit generatinga binary signal when the first threshold potential level is lower thanthe potential level of the low-frequency component of the output signalgenerated as a result of the automatic gain control based on the firstcutoff frequency; a second binary encoding circuit for receiving theoutput signal generated as a result of the automatic gain control in theautomatic gain control circuit and for comparing the output signal witha second threshold potential level; the second binary encoding circuitgenerating a binary signal when the second threshold potential level ishigher than the potential level of the low-frequency component of theoutput signal generated as a result of the automatic gain control basedon the second cutoff frequency; and a gate circuit which is operated inconjunction with the change of the cutoff frequency in the automaticgain control circuit so as to selectively send out either the binarysignal from the first binary encoding circuit at the time of setting ofthe first cutoff frequency in the automatic gain control circuit or thebinary signal from the second binary encoding circuit at the time ofsetting of the second cutoff frequency in the automatic gain controlcircuit.

The background component of the image signal has a low frequency, whilethe character component of the image signal has a high frequency. Thelight-colored component of the image signal has a high potential level,while the dark-colored component of the image signal has a low potentiallevel. For these reasons, the automatic gain control circuit is set atthe first cutoff frequency which is a low frequency, when the imagesignal processor is placed in a light-colored background mode in which adark-colored character or the like is drawn on a light-coloredbackground is imaged. In this case, the automatic gain control isperformed on the light-colored background component (which is alow-frequency component) of the image signal but not performed on thedark-colored character component (which is a high-frequency component)of the image signal. As a result, the potential level of thelight-colored background component is made to be close to the saturationlevel of an amplifier so that the amplitude of the dark-coloredcharacter component generated which is normally a low potential can bemade sufficiently large, as shown in FIG. 2.

When the image signal processor is placed in a dark-colored backgroundmode in which the dark-colored background is imaged, the automatic gaincontrol circuit is set at the second cutoff frequency which is a highfrequency, so that the automatic gain control is performed on both thedark-colored background component (which is a low-frequency component)and light-colored character component (which is a high-frequencycomponent) of the image signal. In this case, the potential level of thedark-colored background component is suppressed to a low level such thatthe amplitude of the light-colored character component generated whichis a normally high potential level can be made sufficiently large, asshown in FIG. 3.

As a result, the potential level of the character component of the imagesignal in each of the modes is made sufficiently higher than the firstor second threshold potential level set in the first or second binaryencoding circuit corresponding to the mode selected. For that reason,the output signal from the automatic gain control circuit is ensured tobe converted into a properly representative binary signal in each of themodes. The cutoff frequency of the automatic gain control circuit isthus changed depending upon the characteristic of the imaged object,thereby making the amplitude of the required signal componentappropriate for enabling a proper encoding.

BRIEF DESCRIPTION OF THE DRAWINGS OF THE PRESENT INVENTION

Other objectives and advantages of the present invention will becomemore apparent from the detailed description of the preferred embodimentswith reference to the accompanying drawing wherein:

FIG. 1 shows a wiring diagram of an image signal processor according toan embodiment of the present invention;

FIG. 2 shows graphs of signal components generated by the processor ofthe present invention in a light-colored background mode;

FIG. 3 shows graphs of signal components generated by the processor ofthe present invention in a dark-colored background mode; and

FIG. 4 shows graphs of signal components generated by conventional imagepickup and printing apparatus.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

An embodiment of the present invention is described below in detail withreference to the drawings attached hereto.

FIG. 1 shows a wiring diagram of an image signal processor according toan embodiment of the present invention. The processor comprises a signalinput circuit 11, an automatic gain control circuit 12, a cutofffrequency changer 13, a first binary encoding circuit 14, a secondbinary encoding circuit 15, a gate circuit 16, and a selection switch17.

The signal input circuit 11 has an operational amplifier OP₁ which is anon-inversion amplifier. An image signal generated by an image pickupelement, such as a charge-coupled device, which is not shown in thedrawings, is supplied to the plus input terminal of the amplifier OP₁through a resistor R₁. The minus input terminal of the amplifier OP₁ isgrounded through a resistor R₂ and connected to the output terminal ofthe amplifier though a resistor R₃.

The automatic gain control circuit 12 has two transistors Tr₁ and Tr₂for performing automatic gain control on the image signal in terms of acutoff frequency, described below, to generate an output signal of aprescribed potential level. The collector of the transistor Tr₁ isconnected to a power supply V_(DC) through a resistor R₄. The emitter ofthe transistor Tr₁ is connected to the base of the other transistor Tr₂whose collector is connected to the plus input terminal of theoperational amplifier OP₁ through a capacitor C₁ and whose emitter isgrounded. The base of the transistor Tr₁ is connected to the outputterminal of the amplifier OP₁ through a capacitor C₂. The base of thetransistor Tr₁ is also connected to the cutoff frequency changer 13 anda diode D₁ whose cathode is grounded through a capacitor C₃ and aresistor R₅. The terminal of the capacitor C₂, which is located oppositethe output terminal of the amplifier OP₁, is grounded through a diodeD₂.

The cutoff frequency changer 13 includes a resistor R₆ and a resistor R₇connected in parallel through an analog switch A-SW. The resistance ofthe resistor R₇ is about ten times as much as the resistor R₆. When theanalog switch A-SW is turned off, a first cutoff frequency, which is alow frequency, is set. When the switch A-SW is turned on, a secondcutoff frequency, which is a high frequency, is set.

The first binary encoding circuit 14 receives the output signal V_(S)generated as a result of the automatic gain control on the image signalby the automatic gain control circuit 12, to perform the binary encodingof the output signal. The first binary encoding circuit 14 has anoperational amplifier OP₂ which acts as a comparator. The output signalVs is applied to the minus input terminal of the amplifier OP₂, which isalso grounded through a resistor R₈ and a capacitor C_(R). The plusinput terminal of the amplifier OP₂ is grounded through a resistor R₁₀and through a resistor R₉ and capacitor C₄. As a result, a firstthreshold potential level V_(A), which is set by integrating the outputsignal V_(S) and making this integrated value slightly lower, is appliedto the plus input terminal of the amplifier.

As shown in FIG. 2, the first threshold potential level V_(A) is set tobe slightly lower than the potential level of the light-coloredbackground component (which is a low-frequency component) of the outputsignal V_(S) generated in a light-colored background mode in which thefirst cutoff frequency, which is the low frequency, is set.

The second binary encoding circuit 15 also receives the output signalV_(S) to perform binary encoding on the output signal V_(S). The secondbinary encoding circuit 15 has an operational amplifier OP₃ which actsas a comparator. The output signal V_(S) is applied to the plus inputterminal of the amplifier OP₃ which is also grounded through a resistorR₁₁ and a capacitor C₅. The minus input terminal of the amplifier OP₃ isgrounded though a resistor R₁₂ and a capacitor C₅. The minus inputterminal of the amplifier OP₃ is also connected to a power supply V_(CC)through a resistor R₁₃.

As a result, a second threshold potential level V_(B), which is set byintegrating the output signal V_(S) and making this integrated valueslightly higher, is applied to the minus input terminal of the amplifierOP₃. As shown in FIG. 3, the second threshold potential level V_(B) isset to be slightly higher than the potential level of the dark-coloredbackground component (which is a low-frequency component) of the outputsignal V_(S) generated in a dark-colored background mode in which thesecond cutoff frequency, which is the high frequency, is set.

The gate circuit 16 has two AND gates, AND₁ and AND₂, for selectingeither the output from the first or the second binary encoding circuits,14 and 15. The output from the first binary encoding circuit 14 isapplied to one input terminal of the AND gate AND₁. A changeover signalfrom the selection switch 17 is applied to the other input terminal ofthe AND gate AND₁ through an inverter INV. The output from the secondbinary encoding circuit 15 is applied to one input terminal of the otherAND gate AND₂. The changeover signal from the selection switch 17 isapplied to the other input terminal of the AND gate AND₂ directly. Onlyone of the outputs from the AND gates, AND₁ and AND₂, is sent as finalbinary data through an OR gate OR.

The selection switch 17 has changeover terminals, a and b, and a commonterminal c. The changeover terminal a is connected to the power supplyV_(CC) through a resistor R₁₄. The common terminal c is grounded. Thechangeover signal is supplied to the cutoff frequency changeover analogswitch A-SW of the automatic gain control circuit 12 and to the gatecircuit 16, through the changeover terminal a, to change the cutofffrequency and to enable one of the AND gates, AND₁ and AND₂.

The operation of the image signal processor will be described in detailbelow. When a dark-colored character or the like drawn on alight-colored background, such as a whiteboard, is to be imaged, theselection switch 17 is first operated to connect the changeover terminala to the common terminal c. This operation puts the processor in thelight-colored background mode. As a result of this operation, thechangeover signal is set at a low potential level, thereby turning offthe analog switch A-SW which sets the first cutoff frequency, a lowfrequency. Also, the AND gate AND₁ for the first binary encoding circuit14 is enabled through the operations of the inverter INV.

When the image signal generated by imaging the dark-colored character orthe like drawn on the light-colored background is then supplied to theprocessor, the automatic gain control circuit 12 performs the automaticgain control on the image signal in terms of the first cutoff frequencyso that the automatic gain control is performed on the light-coloredbackground component (which is a low-frequency component) of the signalbut not performed on the dark-colored character component (which is ahigh-frequency component) of the signal. For that reason, thelight-colored background component (which is a low-frequency component)of the output signal V_(S) has a high potential level, which is close tothe saturation level of the amplifier, and the dark-colored charactercomponent (which is a high-frequency component) of the output signal hasa potential level lower than the light-colored background component andhas a large amplitude, as shown in FIG. 2. The comparator OP₂ of thefirst binary encoding circuit 14 compares the output signal V_(S) withthe first threshold potential level V_(A) which is slightly lower thanthe potential level of the light-colored background component of theoutput signal, so that the circuit generates a binary signal whoselight-colored background component has a low potential level and whosedark-colored character component has a high potential level. The binarysignal is sent out through the AND gate AND₁ and the OR gate OR of thegate circuit 16. Since the dark-colored component (which is thehigh-frequency component) of the output signal V_(S) has the largeamplitude and the low potential level which is sufficiently lower thanthe first threshold potential level V_(A), binary encoding is enabled.

When a light-colored character or the like drawn on a dark-coloredbackground, such as a blackboard, is to be imaged, the selection switch17 is first operated to connect the changeover terminal b to the commonterminal c. This operation puts the processor in the dark-coloredbackground mode. As a result of this operation, the changeover signal isset at a high potential level, and the analog switch A-SW is turned on,thereby setting the second cutoff frequency, a high frequency. Also, theAND gate AND₂ of the gate circuit 16 is made capable of being opened.When the image signal generated by imaging the light-colored characteror the like drawn on the dark-colored background is applied to theprocessor, the automatic gain control circuit 12 performs automatic gaincontrol on the signal according to the second cutoff frequency, which isthe high frequency, so that the automatic gain control is performed onboth the dark-colored background component (which is a low-frequencycomponent) and light-colored character component (which is ahigh-frequency component) of the signal. In other words, the automaticgain control is performed without neglecting the light-colored charactercomponent which contains information concerning the light-coloredcharacter or the like.

As a result, the dark-colored background component (which is alow-frequency component) of the output signal V_(S) is suppressed to alow potential level, which is sufficiently lower than the saturationlevel of the amplifier, as shown in FIG. 3. For that reason, thelight-colored character component (which is a high-frequency componentand has a higher potential level than the dark-colored backgroundcomponent) of the output signal V_(S) has a sufficiently largeamplitude. The comparator OP₃ of the second binary encoding circuit 15compares the output signal V_(S) with the second threshold potentiallevel which is slightly higher than the potential level of thedark-colored background component, so that a binary signal whosedark-colored background component has a low potential level and whoselight-colored character component has a high potential level isgenerated by the circuit. The binary signal is sent out through the ANDgate AND₂ and the OR gate OR. Since the light-colored component has thesufficiently large amplitude and a sufficiently higher potential levelthan the second threshold potential level V_(B), binary encoding isenabled.

While only certain embodiments of the present invention have beendescribed, it will be apparent to those skilled in the art that variouschanges and modifications may be made therein without departing from thespirit and scope of the present invention as set forth in the claimsbelow.

What is claimed is:
 1. An image signal processor comprising:image pickupmeans for generating an image signal corresponding to an image to beprocessed; automatic gain control means, operatively connected to saidimage pickup means, for performing an automatic gain control operationin response to a frequency component of said image signal according towhether said image comprises a light background or a dark background;selection means, operatively connected to said automatic gain controlmeans, for manually setting a cutoff frequency to be used in saidautomatic gain control operation of said automatic gain control means;and binary encoding means, operatively connected to said automatic gaincontrol means and said selection means, for producing binary data ofsaid image signal according to said cutoff frequency set by saidselection means.
 2. The image signal processor as claimed in claim 1,wherein said selection means comprises an analog switch for selectingeither a light background mode or a dark background mode.
 3. An imagesignal processor comprising:image pickup means for generating an imagesignal corresponding to an image to be processed; automatic gain controlmeans, operatively connected to said image pickup means, for performingan automatic gain control operation in response to a frequency componentof said image signal according to whether said image comprises a lightbackground or a dark background; selection means, operatively connectedto said automatic gain control means, for manually setting a cutofffrequency to be used in said automatic gain control operation of saidautomatic gain control means; binary encoding means, operativelyconnected to said automatic gain control means and said selection means,for producing binary encoding signals of said image signal; and gatingmeans, operatively connected to said binary encoding means and saidselection means, for selecting said binary encoding signalscorresponding to said cutoff frequency; said selection means setting oneof at least two cutoff frequencies to be used in said automatic gaincontrol operation; said binary encoding means producing a distinct setof binary data for each cutoff frequency selected.
 4. The image signalprocessor as claimed in claim 3, wherein said selection means comprisesan analog switch for selecting either a light background mode or a darkbackground mode, each background mode having a cutoff frequencyassociated therewith.
 5. The image signal processor as claimed in claim3, wherein said binary encoding means has a distinct output port foreach said set of binary data corresponding to a single cutoff frequency.6. The image signal as claimed in claim 3, wherein said gating meanscomprises:first AND gate means, responsive to said selection means andoperatively connected to an output of said binary encoding means, forselecting said binary encoding signals corresponding to a first cutofffrequency; second AND gate means, responsive to said selection means andoperatively connected to an output of said binary encoding means, forselecting said binary encoding signals corresponding to a second cutofffrequency; and OR gate means, operatively connected to said first andsecond AND gate means, for outputting the selected binary encodingsignals from said gating means.
 7. An image signal processor,comprising:image pickup means for generating an image signalcorresponding to an image to be processed; automatic gain control means,operatively connected to said image pickup means, for performing anautomatic gain control operation in response to a frequency component ofsaid image signal according to whether said image comprises a lightbackground or a dark background; cutoff frequency generating means,operatively connected to said automatic gain control means, forgenerating a cutoff frequency to be used in said automatic gain controloperation; selection means, operatively connected to cutoff frequencygenerating means, for selecting either a light background mode or a darkbackground mode, each background mode causing a different predeterminedcutoff frequency to be generated by said cutoff frequency generatingmeans; binary encoding means, operatively connected to said automaticgain control means and said selection means, for producing binaryencoding signals of said image signal; and gating means, operativelyconnected to said binary encoding means and selection means, forselecting said binary encoding signals corresponding to said cutofffrequency; said cutoff frequency generating means generating one of atleast two cutoff frequencies to be used in said automatic gain controloperation; said binary encoding means producing a distinct set of binarydata for each cutoff frequency selected.
 8. The image signal processoras claimed in claim 7, wherein said selection means comprises an analogswitch for selecting either said light background mode or said darkbackground mode.
 9. The image signal processor as claimed in claim 7,wherein said binary encoding means has a different output port for eachsaid set of binary data corresponding to a single cutoff frequency. 10.The image signal as claimed in claim 7, wherein said gating meanscomprises:first AND gate means, responsive to said selection means andoperatively connected to an output of said binary encoding means, forselecting said binary encoding signals corresponding to a first cutofffrequency; second AND gate means, responsive to said selection means andoperatively connected to an output of said binary encoding means, forselecting said binary encoding signals corresponding to a second cutofffrequency; and OR gate means, operatively connected to said first andsecond AND gate means, for outputting the selected binary encodingsignals from said gating means.
 11. The image signal processor asclaimed in claim 7, wherein said binary encoding means comprises:a firstbinary encoding circuit, operatively connected to said automatic gaincontrol means and said cutoff frequency generating means, for comparingan output signal from said automatic gain control means with a firstthreshold potential level to produce a binary signal corresponding tosaid light background mode; and a second binary encoding circuit,operatively connected to said automatic gain control means and saidcutoff frequency generating means, for comparing said output signal fromsaid automatic gain control means with a second threshold potentiallevel to produce a binary signal corresponding to said dark backgroundmode.